The present invention relates to a shock wave sensor and, more particularly, the invention relates to a shock wave sensor wherein a hollow, carryng shaft is concentrically arranged around another electrode structure, both being connected to piezo electric foil means.
Shock wave sensors are used, for example, for measuring the pressure amplitude of a shock wave as it propagates in a medium, for example, water. A particularly important field of application is the measurement of the pressure in the focal point of shock waves which have been focused.
Qualitatively, shock waves are similar to ultrasonic waves, but these two types of waves differ quantitatively in that shock waves have a very steep leading edge-like onset, a very high pressure peak, and a more or less rapid decay thereafter. Also, shock waves do not have any basic periodicity. This, of course, is due to the fact that the frequency spectrum of shock waves is considerably wider than in the case of ultrasonic waves.
So-called hydrophones are used for detecting ultrasonic waves. They include foils made of a piezo electric polymer, preferably polyvinylidenfluoride (PVF.sub.2 or PVDF). K. C. Shotton, et al. described a hydrophone in "Ultrasonics," 1980, page 123. This hydrophone has a very high sensitivity for the detection of ultrasonic waves. However, it is believed that this kind of hydrophone is unsuitable for the detection of shock waves because the vapor deposited contact devices will be removed by an erosive process caused primarily by cavitation. In fact, these vapor deposited layers will decay very rapidly.
P. A. Lewin describes a hydrophone in "Ultrasonics," September 1981, page 213, but it is analogously defective because shock waves would crack the contact areas of the sensor elements being directly mounted to a rigidly constructed housing. It thus can be seen that ultrasonic hydrophones as per the state of the art are not sufficiently robust for use in the detection of shock waves or shock wave fields.
A shock wave sensor designed specifically for that purpose and for measurements within that field is, for example, described by S. W. Meeks, et al. in the "Journal of the Acoustical Society of America," 1984, page 1010. Therein a thick PVF.sub.2 foil, being about 500 micrometers thick, is used and suspended together with several disks within a three-legged frame. The thickness of the foil, however, reduces the band width of the sensor considerably and renders it sensitive to the detection of transverse waves.